The present invention relates to an angular error detecting device utilizing an antenna for receiving radio signals from a target and in which the directional angular error between the direction of the antenna and the direction of the target is detected.
In the case where a polarized wave from a target has a constant polarization such as polarization or linear polarization, a simple method has been known in the art to detect the directional angular error and a variety of angular error detecting devices using this technique has been put to practical use. However, recently it has been required to provide an angular error detecting device which operates satisfactorily even in the case where a target emits or reflects waves which has a nonconstant polarization such as elliptical polarization or which have a polarization inclination which changes with time. This device is intended for use in a system of compensating cross polarization due to rain, etc. in the propagation path.
FIG. 1 shows an example of a conventional angular error detecting device of this type. In this device, a signal from a target is received by an antenna 1, and only a high order mode component excited in a circular waveguide is picked up by higher mode couplers 2 and 3. The higher mode couplers 2 and 3 are connected in an orthogonal relation. The couplers 2 and 3 are, for instance, a TE.degree..sub.01 mode coupler and a TM.degree..sub.01 mode coupler, respectively. The polarization configuration and polarization plane of the fundamental mode component are converted by polarization converters 4 and 5. In this example, the converters 4 and 5 are a 90.degree. phase difference plate (PDP) and a 180.degree. phase difference plate 5, respectively.
The signal subjected to polarization conversion is separated into two polarization components E.sub.xc and E.sub.yc which are orthogonal to each other by a polarization coupler 6. The component E.sub.xc is a main polarization component and the component E.sub.yc is an orthogonal polarization component. The component E.sub.xc is coupled to a phase locked loop (hereinafter referred to as "a PLL") at one input of a phase detector (hereinafter referred to as "a PSD") 7, a loop filter 8 and a voltage-controlled oscillator (hereinafter referred to as "a VCO") 9. The output of the VCO 9 is maintained different by 90.degree. in phase from the component E.sub.xc. On the other hand, the component E.sub.yc is subjected to detection by a PSD 17 using the output of the VCO 9 as a reference. The output of the VCO 17 is applied through an amplifier 18 to an electric motor 19 to drive the aforementioned polarization converter 5 and to drive the polarization converter 4 through a gear box 20.
The higher mode components from the higher mode couplers 2 and 3 are combined by an orthogonal polarization composer 33. The output of the composer 33 is applied through polarization converters 4 and 5 to a polarization coupler 36 where it is divided into two orthogonal components E.sub.XD and E.sub.YD. These components are subjected to synchronous detection by PSD's 37 and 48 using the output of the VCO 9 directly and the output of the VCO 9 applied through a 90.degree. phase shifter 10 as references. The outputs of the PSK's 37 and 38 are applied to low-pass filters 38 and 48 so that only dc components are obtained. As a result, two angular error voltages .DELTA.X and .DELTA.Y are provided.
The above-described operation will be further described using mathematical expressions.
FIG. 2 is a diagram showing relationships between a target, a tracking antenna with the angular error detecting device as described above and a polarized wave. In FIG. 2, reference character E designates the tracking antenna with the device and T the target. A coordinate system is used having as its origin the intersection of the central axis of the beam of the antenna and a plane including the target T. In FIG. 2, an angle .DELTA..theta. between EO and ET is the angular error of the tracking antenna and a value .phi. is an angle corresponding to the orientation of the angular error and the inclination of the latter with respect to the X-axis. With respect to the configuration of arrived polarized waves as shown in FIG. 2, "E.sub.max " and "E.sub.min " represent the major axis electric field and the minor axis electric field of elliptical polarized waves respectively, and ".gamma." is the inclination of the direction of the electric field E.sub.max with respect to the X-axis.
In the case where the tracking antenna is provided with the angular error detecting device as shown in FIG. 1 under the conditions as shown in FIG. 2, the electric fields E.sub.XA and E.sub.YA at the input of the polarization converter 4 can be represented by the following expressions: EQU E.sub.XA =E.sub.max cos .gamma.-j E.sub.min sin .gamma. EQU E.sub.YA =E.sub.max sin .gamma.+j E.sub.min cos .gamma.. (1)
The values E.sub.XA and E.sub.YA are the X component electric field and the Y component electric field at the point where the values E.sub.XA and E.sub.YA are determined after the X-Y coordinate in FIG. 2 is moved in the direction OE. The same is applicable to E.sub.XB, E.sub.YB, E.sub.YC, E.sub.XD and E.sub.YD. If the angles of the polarization converters 4 and 5 are represented by .alpha. and .beta., respectively, then E.sub.XC and E.sub.YG are: EQU E.sub.XC =-E.sub.max sin (.alpha.-2.beta.) sin (.alpha.-.gamma.) +E.sub.min cos (.alpha.-2.beta.) sin (.alpha.-.gamma.) -jE.sub.max cos (.alpha.-2.beta.) cos (.alpha.-.gamma.) +jE.sub.min sin (.alpha.-2.beta.) cos (.alpha.-.gamma.) EQU E.sub.YC =-E.sub.max cos (.alpha.-2.beta.) sin (.alpha.-.gamma.) -E.sub.min sin (.alpha.-2.beta.) sin (.alpha.-.gamma.) -jE.sub.max sin (.alpha.-2.beta.) cos (.alpha.-.gamma.) +jE.sub.max cos (.alpha.-2.beta.) cos (.alpha.-.gamma.). (2)
In the case where the device shown in FIG. 1 receives a linearly polarized wave, the relation .alpha.=2.beta. should be established by the gear box. In this case, control for .alpha.=.gamma. is automatically carried out by the PSD 17, the amplifier 18 and the motor 19. In this operation, EQU E.sub.XC =-j E.sub.max EQU E.sub.YC =j E.sub.min. (3)
The angular error signals outputted by the higher mode couplers 2 and 3 are combined by the orthogonal polarization composer 33 into E.sub.XB and E.sub.YB. These can be expressed as follows, when .DELTA..theta. is small, EQU E.sub.XB =k.DELTA..theta.{E.sub.max cos (.gamma.-.phi.)-j E.sub.min sin (.gamma.-.phi.)} EQU E.sub.YB= k.DELTA..theta.{E.sub.max sin (.gamma.-.phi.)-j E.sub.min sin (.gamma.-.phi.)}, (4)
where k is a proportional constant.
The angles of the polarization converters 34 and 35 are the same as those of the polarization converters 4 and 5, i.e. .alpha. and .beta.. Therefore, under the conditions .alpha.=2.beta. and .alpha.=.gamma., the outputs E.sub.YD and E.sub.YD of the polarization coupler 36 are: EQU E.sub.XD =k.DELTA..theta.{E.sub.min sin.phi.=j E.sub.max cos.phi.} EQU E.sub.YD =k.DELTA..theta.{E.sub.max sin.phi.-j E.sub.min cos.phi.}. (5)
Therefore, if only a component of the output E.sub.XD, which is in phase with the output E.sub.XC, is detected by PSD 37, the following expression (6) is obtained: ##EQU1## Similarly, if only a component of the output E.sub.YD which is orthogonal to E.sub.XC is detected by the PSD 47, then the following expression (7) is obtained: ##EQU2## The values .DELTA.X and .DELTA.Y given by the expressions (6) and (7) are components in the X direction and in the Y direction, respectively, of the angular error .DELTA..theta., as is apparent from FIG. 2.
In the case where the device as shown in FIG. 1 is used to receive circularly polarized waves, the polarization converters 4, 5, 34 and 35 are not driven and .alpha. is set to -.pi./4 and .beta. is set to zero.
In this case, ##EQU3##
Therefore, EQU E.sub.XD /E.sub.XC =k.DELTA..theta..THETA..sup.-j.phi. =k.DELTA..theta. (cos.phi.-j sin.phi.). (10)
Detecting E.sub.XD with the PSD 37 and the PSD 47 with E.sub.XC as a reference, ##EQU4## are obtained.
In this case, the armature of a switch 41 in FIG. 1 is connected as shown, and in the case of a linearly polarized wave, the armature is set to the E.sub.YD side.
As is apparent from the above description, in the system shown in FIG. 1, the angular error voltages X and Y are obtained by suitably switching the angular error detecting circuit according to the conditions of received polarized waves. However, the circuits in the device in FIG. 1 must be switched separately according to whether circularly polarized waves or linearly polarized waves are being received. That is, the device is disadvantageous in that, when polarization of the arriving waves alters quickly, not only the operation becomes intricate, but also it is difficult to maintain signal reception satisfactorily. In other words, the device suffers from a drawback in that all the components of arrived polarized waves cannot be obtained for E.sub.XC. For instance, if a counterclockwise circularly polarized wave arrives while the device is receiving a clockwise circular polarized wave, then E.sub.XC =0 occurs (E.sub.max =-E.sub.min in the expression (8)). In this case, the device may not be able to receive the signal.